Faulty Coolant sensor, Ford Mondeo deisel 2000cc year 2004?

The temperature sender unit is faulty can you please advise what size is the hexagon i.e. spanner size / socket required so I can remove and fit a new temperature sender in the cylider head. The unit comes complete with the electrical wire crimped to the sender unit making it difficult to find a suitable tool.

Can you please advise what tool is required to do this or tips on how to replace it.

Thanks your assistance is appreciated.